Quick heating cathode for electron discharge device



` Nov. 12., 1957 A. P. SWEET, JR. 2,813,227

'QUICK HEATING CATHODE FOR ELECTRON DISCHARGE DEVICE Filed Oct. 19, 1951 United rates Patent QUICK HEATING CATHODE FOR ELECTRON DISCHARGE DEVICE Arthur P. Sweet, Jr., Lancaster, Pa., assigner to Radio Corporation of America, a corporation of Delaware Application October 19, 1951, Serial No. 252,150

4 Claims. (Cl. 315-107) This invention pertains to an electron tube and more particularly to a quick-heating emitter or cathode construction for such tube and its associated circuit.

In many types of electronic equipment, it is desirable to reduce power consumption during standby periods by turning oli the heater or filament supply voltages of the electron tubes. However, in such equipment, it is also desirable to have the equipment return to normal operation in as short a time as possible after the heater supply voltage has been turned on. In the past, it has been the practice, in designing electron tubes for such applications, to use a directly heated filament type emitter, instead `ot` the indirectly heated type, because of its low heat capacity and consequent short heating time. However,` such filament type emitters have many disadvantages such as complicated assembly, vibration and microphonic problems, short life, and mechanical difficulties in maintaining the lilament position accurately enough to allow close grid-filament spacings without shorting troubles.

Accordingly, the main object of my invention is to provide an indirectly-heated cathode or emitter of large surface area which will heat up to operating temperature in a very short period of time.

A further object of my invention is to provide an indirectly heated emitter which will heat up quickly and which will be of rugged construction.

In general, I accomplish the aforesaid objects and overcome all of the aforementioned difliculties by passing an initial surge of current directly through the cathode sleeve of an indirectly heated cathode. As the cathode sleeve reaches operating temperature, it is disconnected from the voltage source and then a conventional type heater maintains the cathode at the required temperature there-- after.

My invention will be described with continuous reference to the accompanying drawing wherein Figs. 1 and 2 represent two embodiments partially in section of my invention, and Figs. 3 and 4 show further modications of my invention.

Referring to Fig. l, I have shown some of the Components of an electron discharge device having an envelope 5 and within which are mounted the electrodes including a cathode assembly which includes a metal cathode sleeve` 8 here represented as` being cylindrical in form (although such form is not essential) and having an emissive coating thereon. The cathode sleeve is surrounded by a grid 7 and an anode 9 in a conventional manner. These last two electrodes may be supported on side rods in the conventional manner.

In accordance with my invention, there is extended across the top of the cathode sleeve 8 and forming a part thereof a diaphragm 1` carrying a contact button 2 for a purpose which will be later explained. The cathode sleeve 8 is supported by a mounting cone 12 which in turn is positioned on an insulated support 14. A conductor rod 4, having a contact button 3 thereon, extends upwardly from the said support into or Within the cathode ICC sleeve 8 with which it is adapted to make Contact through contact ofthe buttons 2 and 3 mounted on the diaphragm and cathode, respectively.

An insulated heater wire 6 is coiled around the conductor rod 4 and is electrically connected at its ends to the said rod and to the mounting cone 12, respectively. A heater voltage supply 18 provided with a switch 16 is connected to the bottom 20 of the conductor rod and to the mounting cone 12.

The operation of my device is as follows:

The cathode sleeve is constructed of a material having a higher resistance and a higher coeicient of expansion than the material of the conductor rod so that when current is flowing in the two members, the cathode sleeve heats more quickly and expands faster than. the conductor rod. When the cathode sleeve is cold, the buttons 2 and 3 are in contact with each other but when the cathode sleeve is heated by current iiow therethrough, contact between the buttons is broken due to the uneven expansion of the cathode sleeve and conductor rod.

To heat the tube, the switch 16 is closed, whereupon the heater voltage is applied to the device and the heater current divides into two parallel paths. ries the normal operating current through the heater wire 6 and the conductor rod 4 and the other path carries a large surge of current through the mounting cone 12, the cathode sleeve 8, contact buttons 2 and 3 and conductor rod 4, thus heating such parts. Due to the aforementioned uneven expansion of the rod and sleeve, contact between the buttons 2 and 3 is broken and the sleeve is cut oil? from the heater supply voltage. However, by the time that this happens, the cathode sleeve 8 and the heater `6 have reached operating temperature and the heater can maintain the cathode at normal operation thereafter. On the other hand, when switch 16 is again opened and the heater current is cut off, the sleeve will cool and contract to its original position, whereupon the buttons 2 and 3 will again be brought into contact with each other and the components of the tube will then be ready for the next cycle of operation.

A second embodiment of my invention is shom in Figure 2 wherein the cathode sleeve 8 is provided with an emissive coating 10 and is positioned on the mounting cone 12, A conductor rod 4 having a heater wire 6 wound therearound, ismounted within the cathode sleeve and is arranged to contact the top 34 of the sleeve. A grid 7 and an anode 9 surround the sleeve in conventional fashion.

For impressing voltages on the device, a transformer 30 is provided. Such transformer essentially comprises a single primary 22 and a two-part secondary winding 24 and 216 for supplying voltages to the heater wire 6 and `the cathode sleeve 8, respectively, the part 24 of the secondary winding being connected to the heater wire and the part 26 of such winding being connected to the sleeve.

A time delay switch 28 is connected in the current path through the cathode sleeve 8. The operation of this switch is well known in the art, and it will not be described any further than to say that the switch is adjusted to cut off current flow through the sleeve as soon as the heater and cathode reach normal operating temperature.

It is clear that there are many embodiments: in which my basic invention may be practiced. Another variation is shown in Fig. 3 wherein a cathode sleeve 44 is provided at its lower end with an L-shaped conductive bracket 54 which carriesa contact button 46. A mica or ceramic separator or spacer 56 provides support for a similar L-shaped conductive bracket 50 which also carries a contact button 48. The upper end of the cathode sleeve 44 is fixed against movement by the bracket One path car- 39 which is supported by an insulating spacer 58. Conventional conductive support rods 40 and 43 are positioned within the tube by means of the separators, 56 and 58`and the rod 40 is electrically connected to the' cathode sleeve by means of a connector 42. The tube is supplied with a heater filament wire 41 and a voltage source 52 which is connected across the filament and across a path 'formed by the-rod 40, the cathode sleeve 44, the bracket 54, the contact buttons 46 and 48, and the bracket 50. A grid 45 and an anode 47 are positioned around the sleeve in well known fashion.

When the tube is not in operation, the supply voltage is not applied to the tube and all of the component parts of the tube are cold. In this condition, the buttons 46 and'4S-make contact with each other. However, when the voltage source is connected to the tube, current flows through-the filament and through the path including the sleeve as described above. The current flow causes heating and asithe sleeve becomes heated, it expands until the connection between the contact buttons is broken and the Vsleeve isthereby disconnected from Vthe voltage source. By theV time such contact is broken, thecathode has reached operating temperatureV and the tube continues to functionin its normal fashion.

A further embodiment, illustrated in Fig. 4, shows an arrangement for heating av cathode 60 which comprises a two-part filament made of a normal heater 62 and a high power heater 64. A voltageV supply source 68 is connected across each heater and atime delay switch or other suitable device is placed in series with the high power section. When the tube is energized initially,- current ows through both filaments and the high power heater stimulates the cathode to emit in a very short period-of time. After a predetermined time, and generally after the cathode has reached operating temperature, then time delayA switch functions to disconnect the heater 64 from .-.the voltage source and the tube continues to function in normal fashion by means of the normal heater.

My, invention has-many advantages over prior art methods of providing quick heating filament type emitters. It provides a rugged self supporting structure which eliminates the need 'for tensioning devices and which reduces microphonics and vibration problems. It allows construction of a cathode having a large area with resultant high peak currents. The tube elements can be constructedV with close tolerances with the result that the close grid-cathode spacings particularly necessary in high frequency electron tubes are achieved. The cathode sleeve wall thickness can be varied to control thepeak surge current, heat capacity and heating time. The size, material, and shape of the center conductor rodl can be varied to control the time during which the contacts remain closed and the material, shape and all thickness of the supporting cone can be varied to provide heat isolation of the cathode during normal operation and if desired, additional end heating during the initial heating period. A particular advantage of the embodiment shown in Fig. 2 is that the cathode sleeve may be insulated from the heater if such construction is desired.

What is claimed is:

, 1. A cathode assembly comprising a metal sleeve having a closed end, a heater assembly within said sleeve comprising a conducting rod, said rod having a lower coefficient of expansion than said sleeve, and a common insulating support engaging the other end of said sleeve and' one end ofsaid rod and supporting said rod and sleeve in axial parallel relation, said heater assembly including `a heater wire supported on and electrically engagingY the-other end of said rod, said rod and sleeve having lengths disposing said other end of the rod and said closed end of the sleeve in contacting relation when said sleeve is unheated, whereby connection of said heater, said rod, and said other end of said sleeve to a source of electrical energy first causes direct heating of said sleeve and said heater wire to rapidly heat said sleeve until said sleeve expands to disengage said closed end of said sleeve from said other end of the rod, and then causes direct heating of said heater wire only.

2. A cathode assembly for an electron tube comprising a metal sleeve having an electron emitting coating on the outer surface thereof, a heater assembly including` a metal rod, anda-common insulating support engaging' adjacent portions of said sleeve and rod restrainingrelative movement thereof and normally disposinganother portion of said rod in contact with said sleeve, a heater wire supported on said VVrod and providing one path for electrical current, said rod and said sleeve providing another path for electrical current, whereby said sleeve is adapted to be quickly heated by electrical current traversing said two paths, said sleeve having a higher coefficient of expansion than said rod, whereby said another path is broken when said sleeve reaches a predetermined temperature, said heater wire being adapted to continue the heating of said sleeve to maintain said predetermined temperature thereof after said another path has been broken.

3. A cathode and a heater assembly adjacent to said cathode, said heater assembly comprising a relatively thickrod having one end in fixed relation with respect to said cathode, and a heater wire in electrical contact with the other end of said rod, the other end of said rod normally being in contact with said cathode providingrone path for electrical energy through said cathode and rod, said'heater wire and said rod providing another path'for electrical energy, said rod and said cathode being made of materials having different coefficients of expansion, whereby relative movement takes place between said other end of said rod and said cathode when said cathode reaches a predetermined temperature for interrupting said one path, and said heater wire alone continues to heat said cathode.

4. A cathode assembly comprising a metal sleeve, a heater assembly within said sleeve and comprising a metal member and a heater wire supported thereon, a common insulating support engaging adjacent portions of said sleeve and member restraining relative movement thereof and normally disposing another portion of said member incontact with said sleeve, said wire providing one path forelectrical energy,.said metal niemberand said sleeve normally providing a second path for electrical energy, whereby said sleeve is adapted to be quickly heated to a predetermined temperature by electrical energy traversing said two paths, said sleeve and metal member are each made of'a material having a different coefficient of expansion from the material of the other, said sleeve and said metal member mounted to break said contact when said sleeve reaches said predetermined temperature, said heater wire being adapted to continue the heating of said sleeve to maintain heating of said sleeve after said second path has been broken.

References Cited in the file of this patent UNITED STATES PATENTS 1,374,647 Gimingham Apr. 12, 1921 1,787,300 Alexanderson Dec. 30, 1930 1,815,483 Ruben July 2l, 1931 1,822,360 Miesse Sept. 8, 1931 1,943,523 Giard et al. c Jan. 16, 1934 2,522,259 Fay Sept. 12, 1950 FOREGN PATENTS 214,036 Great Britain Apr. 17, 1924 

